Patent GB-1035708-A (GB)
Title: Improvements in and relating to a television circuit arrangement including a camera tube
IPC: H04N5/235
Filed: 1964-04-13
Abstract: 1,035,708. Automatic control systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. April 13,1964 [April 16,1963],No. 15134/64. Heading G3R. [Also in Division H4] The invention relates to a television camera incorporating a camera tube 5 in front of which is an adjustable iris diaphragm 2 and is concerned with controlling the camera operation to compensate for variations in intensity of the impinging light by control of both the voltage applied to an electrode 4 and the size of the aperture of diaphragm 2. The principle of the invention is illustrated in Fig.2 which shows a plot of the size of the diaphragm aperture O and the electrode voltage Va against light intensity Li. As the light intensity increases the electrode voltage is decreased as shown in the full line curve until the intensity reaches the value Lib. At this voltage level Vag, the level is held constant while the iris diaphragm aperture is progressively reduced from its maximum value on to its minimum value Or. Beyond this point the voltage of electrode 4 is again reduced. The output from the camera amplifier 7 is applied to a D.C. restorer circuit 10,11 which stabilizes the level of the blanking porches at earth. The signal is then applied to a peak detector 8,13, 48 which derives a voltage proportional to the level of the signal exceeding a bias level which is applied to the diode. The resulting signal is applied through an amplifier 12 to a lamp 14 which illuminates three photo resistors 15, 16 and 17. The resistor 16 is connected as part of a potentiometer with a fixed resistance 18 and controls the potential applied to electrode 4. As the camera output increases in response to increasing input light intensity the voltage applied to electrode 4 is reduced as the value of the resistance of photo resistor 16 is gradually reduced. Photo resistor 15 forms part of a bridge circuit together with resistors 20,21 and 22, the bridge being arranged to be balanced when the light output from lamp 14 is at a level such to produce a voltage level Vag on electrode 4. Thus if the light input to the camera exceeds the level Lib the bridge becomes unbalanced and causes a voltage to be applied to a motor 25 which operates to decrease the size of the aperture in iris diaphragm 2. The motor drives the iris diaphragm until the light output from lamp 14 reduces the voltage applied to electrode 4 to the level Vag. As the light input to the camera increases from the level Lib to the level Lie the voltage applied to electrode 4 is stabilized at the level Vag by progressive reduction in the size of the iris diaphragm aperture. The iris diaphragm controls a pair of switches 26, 27 which are normally closed over the normal operating range of the iris diaphragm. When the diaphragm reaches its minimum aperture switch 26 is opened and the current ceases to flow to motor 25. The operation of the bridge is such that increasing light output from lamp 14 tends to cause point 24 to go positive of point 23. When switch 26 is opened rectifier 28 becomes operative in the circuit and prevents current flowing through motor 25. If now the light intensity reduces the light output from lamp 14 will reduce and point 24 will go negative of point 23. Under these conditions a voltage can flow from point 23 through motor 25 and rectifier 28 to cause the motor to rotate in the opposite direction and progressively open the iris diaphragm. When the diaphragm reaches its maximum opening switch 27 opens to introduce rectifier 29 in the circuit and prevent further flow of current through motor 25. When the light intensity again increases point 24 will go positive of point 23 and current can flow through motor 25 and rectifier 29 in a direction to once again reduce the size of aperture 2. The circuit thus operates to establish operation according to the curves shown in Fig. 2. The third photo resistor 17 is connected in a potentiometer circuit with fixed resistor 33 to supply an automatic-gaincontrol voltage over lead 34 to amplifier 7. The connection from the output of amplifier 7 to the detector circuit 8, 13, 48 includes a resistor shunted by a switch 35. When the switch 35 is closed to short circuit resistor 36 the detector functions as a conventional peak detector. If the switch is opened to introduce the resistor into the circuit the rectifier ceases to function as a peak detector and produces an output dependent upon the grey levels in the signal. A second embodiment is disclosed with reference to Fig. 4 (not shown) which utilizes transistors. Lamp 14 and the photo resistors are dispensed with and the output voltage from the detector operates directly into the transistor circuit. The invention is described primarily as applied to a television camera which incorporates a vidicon tube, but is stated to be applicable to other types of tubes.